A shape memory alloy has a remarkable shape memory function caused by a martensitic transformation and a martensitic reverse transformation, thereby being useful as an actuator material, etc. An actuator formed by a shape memory alloy is usually driven by heat, with a martensitic transformation by cooling, and a martensitic reverse transformation by heating. In the shape memory alloy, a transformation temperature during cooling is generally higher than a reverse transformation temperature during heating. The difference between the transformation temperature and the reverse transformation temperature is called “temperature hysteresis.” In a thermoelastic martensitic transformation with a small temperature hysteresis, a large shape recovery strain of up to about 5% is usually obtained. However, because a heat-driven actuator has a cooling speed determined by heat dissipation, its response speed is slow.
Thus, attention has been paid to ferromagnetic shape memory alloys such as Ni—Co—Al alloys, Ni—Mn—Ga alloys, etc. undergoing a twinning deformation of a martensite phase induced by a magnetic field. A magnetic-field-induced strain can be obtained in the ferromagnetic shape memory alloy, which is thus promising as an actuator material having a high response speed.
JP 2002-129273 A proposes an actuator member formed by a ferromagnetic shape memory alloy having a composition comprising 5-70 atomic % of Co, 5-70 atomic % of Ni, and 5-50 atomic % of Al, the balance being inevitable impurities, which has a single-phase structure of a β phase having a B2 structure, or a two-phase structure comprising a γ phase and a β phase having a B2 structure. However, even if a magnetic field were applied to this ferromagnetic shape memory alloy, its martensitic transformation temperature would not drastically change, being difficult in causing a martensitic transformation and a martensitic reverse transformation in a practical temperature range. Accordingly, magnetically driving actuators formed by this ferromagnetic shape memory alloy would not have sufficient characteristics at room temperature. Thus, a strong magnetic field is now applied to a ferromagnetic shape memory alloy having only a martensite phase to cause a large twin-crystal magnetostriction. This method is, however, disadvantageous in failing to obtain a large strain unless the ferromagnetic shape memory alloy is a single crystal.
JP 10-259438 A proposes a Ni—Mn—Ga alloy showing a shape memory effect by a magnetic field at an daily life temperature, which has a chemical composition of Ni2+xMn1−xGa, wherein 0.10≦x≦0.30 by mol, and a martensitic reverse transformation-finishing temperature of −20° C. or higher. However, this Ni—Mn—Ga alloy does not have a sufficient shape recovery strain.
As a Mn alloy exhibiting larger strain than that of the Ni—Mn—Ga alloy, JP 2001-279360 A proposes a Mn alloy represented by the general formula of MnaTbX1−a−b, wherein T is at least one selected from the group consisting of Fe, Co and Ni, X is at least one selected from the group consisting of Si, Ge, Al, Sn and Ga, and a and b are numbers meeting 0.2≦a≦0.4, and 0.2≦b≦0.4, and undergoing a martensitic transformation, whose reverse transformation-finishing temperature is in a range of −20° C. to 300° C. However, this Mn alloy fails to exhibit a large strain, because of a magnetic field-induced transformation from a paramagnetic parent phase (matrix phase) to a ferromagnetic martensite phase.
As a magnetic shape memory alloy exhibiting large strain ratio and displacement by crystal transformation, JP 2001-279357 A proposes a magnetic shape memory alloy represented by the general formula of M12−xM2yM3z, wherein M1 is Ni and/or Cu, M2 is at least one selected from the group consisting of Mn, Sn, Ti and Sb, M3 is at least one selected from the group consisting of Si, Mg, Al, Fe, Co, Ga and In, and x, y and z are numbers meeting 0<x≦0.5, 0<y≦1.5, and 0<z≦1.5, having a Heusler structure, and causing a martensitic transformation and a magnetic-field-induced martensitic reverse transformation. This reference describes that the alloy's shape changes by a magnetic field, but all Examples are directed to a magnetic field-induced transformation occurring after a temperature transformation, no Examples showing a martensitic reverse transformation caused only by the change of a magnetic field.
Proposal has been made to provide a thermomagnetic-driving device utilizing the phenomenon that a ferromagnetic shape memory alloy changes between a ferromagnetic state and a paramagnetic state depending on the temperature change. JP 10-259438 A and JP 2002-129273 A describe that ferromagnetic shape memory alloys having compositions optimized to show a magnetic transformation at a daily life temperature are used for actuators. However, there is no sufficient energy conversion efficiency in the magnetic transformation between a ferromagnetic state and a paramagnetic state.
Proposal has also been made to utilize a ferromagnetic shape memory alloy as a magnetic freezer. Magnetic freezing utilizes a magnetocaloric effect, which is a phenomenon that when a magnetic body is isothermally magnetized from a paramagnetic state to a ferromagnetic state, causing a magnetic entropy change due to the difference in the degree of freedom in electromagnetic spin, and then adiabatically deprived of a magnetic field, the temperature of the magnetic body decreases.
As a magnetic material performing magnetic freezing by a relatively weak magnetic field in a room temperature range, JP 2002-356748 A proposes (a) a magnetic-freezer comprising at least one metal selected from the group consisting of Fe, Co, Ni, Mn and Cr in a total amount of 50-96 atomic %, at least one metal selected from the group consisting of Si, C, Ge, Al, B, Ga and In in a total amount of 4-43 atomic %, and at least one metal selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb in a total amount of 4-20 atomic %, and (b) a magnetic-freezer comprising at least one metal selected from the group consisting of Fe, Co, Ni, Mn and Cr in a total amount of 50-80 atomic %, and at least one metal selected from the group consisting of Sb, Bi, P and As in a total amount of 20-50 atomic %. However, these magnetic freezers show sufficient magnetic entropy change only at −40° C. or lower, being not usable in practical applications. Accordingly, magnetic freezers exhibiting sufficient magnetic entropy change at around room temperature are desired.